Paredes had been convicted of killing three members of a rival gang in 2000.

“Father please accept my soul. I am sorry,” were among his last words, Clark said.

Authorities did not disclose the drug protocol used in the execution. There has been considerable controversy in recent months over US executions involving lethal injections going wrong.

US states using the death penalty are facing critical shortages of the anesthetic pentobarbital after European firms stopped supplying it.

The shortage has prompted many states to turn to new, less regulated, pharmaceutical suppliers instead.

Death penalty opponents allege that three recent US executions — which left inmates suffering far longer than the typical 10-minute timeframe — amounted to a form of torture or the “cruel and unusual” punishment forbidden by the US Constitution.

Eighteen US states have abolished the death penalty, but the 32 others — and the federal government — maintain the practice. Surveys indicate that it retains majority support among the US public.

In the Missouri case, a stay was granted by the Supreme Court two hours before death row inmate Mark Christeson, 35, was scheduled to be put to death. The vote by the court was 6-3.

The court agreed to study his appeal, based on the argument that his court appointed lawyers did not represent him properly by missing deadlines for filing appeals.

Christeson was convicted in 1998 of killing Susan Brouke by cutting her throat in front of her two children aged 9 and 12, drowning them in a pond as she watched, and then throwing her into the water while she was still alive.